[1.0.15] Oblivion Clerics doesn't prevent being surprised

Currently the Oblivion Cleric 2nd level ability "Peaceful Rest" doesn't prevent being caught by surprise when travelling overworld. I know some encounters are scripted, but this has been happening on normal random encounters as well. I double checked the "powers" section to see if it had to be turned on in some way, but there wasn't anything there.